1. Field of the Invention
This invention relates to a process for making shaped semiconductor nanocrystals and a resulting product therefrom. More particularly, this invention relates to a process for controlling the shape of Group II-VI semiconductor nanocrystals during the formation of same.
2. Description of the Related Art
Semiconductor nanocrystals, such as Group II-VI nanocrystals are formed by dissolving a Group II precursor and a Group VI precursor in a solvent and then applying heat to the solvent and the precursors therein. For example, Group II-VI semiconductor nanocrystals may be formed by dissolving a dialkyl of the Group II metal and a Group VI powder in a trialkyl phosphine solvent at ambient temperature, and then injecting the mixture into a heated (340.degree. C.-360.degree. C.) bath of tri-octyl phosphine oxide (TOPO).
While the just described process is capable of producing Group II-VI semiconductor nanocrystals, the results can be somewhat erratic in terms of average particle size and size distribution. While it is not certain why the process is not always reproducible, it is believed that impurities in the technical grade (90% pure) TOPO may be adversely influencing the reaction. However, substitution of pure TOPO for the technical grade TOPO has also been unsatisfactory, particularly when control of the shape of the particle growth is also desired, apparently because the pure TOPO binds too weakly to the growing crystallites and only weakly associates with the Group II metal to act as a growth retardant, resulting in the growth of spheres rather than any other desired shapes. Apparently, the presence of impurities in the technical grade TOPO result in the erratic success of Group II-VI semiconductor nanocrystal growth in technical grade TOPO.
The growth of rod-like semiconductor crystals has been reported by W. Z. Wang et al. in "Synthesis and Characterization of MSe (M=Zn, Cd) nanorods by a New Solvothermal Method", Inorganic Chemistry Communications 1999 Mar, Vol. 2, N3:83-85. However, the rod-shaped crystals are out of the confinement region, i.e., are not of nanocrystal dimensions.
Alivisatos et al. U.S. Pat. No. 5,505,928, by one of us with another, and assigned to the assignee of this invention, and the subject matter of which is hereby incorporated by reference, describes a process for forming Group Ill-V semiconductor nanocrystals wherein size control is achieved through use of a crystallite growth terminator which controls the size of the growing crystals. Crystallite growth terminators are said to include a nitrogen-containing or a phosphorus-containing polar organic solvent having an unshared pair of electrons. The patent states that this growth terminator can complex with the metal and bind to it, thereby presenting a surface which will prevent further crystal growth.
Since Group II-VI semiconductor nanocrystals are of interest for use in optical displays, as well as in biological applications, it would be desirable to provide a process for control of shape as well as size growth of such Group II-VI semiconductor nanocrystals wherein the shape (aspect ratio), as well as the particle size, growth rate, and particle size distribution, can be reproducibly controlled, whereby, for example, spheres or rods of semiconductor nanocrystals of controlled dimensions could be formed in a controllable and repeatable manner.